Electrical relays are switches used to electronically turn relatively heavy loads on or off. They are triggered by small amounts of power from an external source (usually a switch operated by the user), but can pass larger amounts of power to the load. This reduces the cost and physical requirements of the switch and can help automate some processes. In the context of nomadic living, a typical setup looks like this: switch (and house power) --> relay --> load.
The most common relay that nomads are exposed to is the solenoid for charging house batteries from the vehicle's alternator. When the vehicle is turned on, power activates the solenoid and connects the house and vehicle electrical systems. See battery isolation.
Relays are also useful for powering loads from low-voltage disconnect or charge controller LOAD outputs. This allows powering larger loads than the controller is rated for.
Relays come in two basic types:
- Normally Open (NO) - the output circuit is open/incomplete when the relay is resting. No power flows. The controlled device is off until the relay is triggered. This is more common.
- Normally Closed (NC) - the output circuit is closed/complete when the relay is resting. Power flows. The controlled device is on until the relay is triggered. This is less common.
Normally it takes power from the trigger source to operate the relay. A latching relay, however, moves from position to position and locks there even when power is removed.